Double-rolled tubes, having excellent appearances similar to those of copper tubes and excellent thermal characteristics, as well as high strength and toughness due to steel, have been used in the fields of connection tubes for various compressors and brake tubes of vehicles.
Double-rolled tubes are described in detail in, for example, "TETSU-TO-HAGANE", No. 1, p. 130 (1980). A typical method for making a double-rolled tube will now be described in brief. Using a cold-rolled steel sheet having a thickness of approximately 0.30 mm, both faces of the steel sheet are electroplated with copper. Next, the steel sheet is furled such that the rolling direction of the steel sheet is parallel to the central axis of the tube. The steel sheet is furled double so that the thickness of the tube is double that of the steel sheet. The tube is heated to a higher temperature than the melting point of copper for "self-brazing" which represents bonding of the steel sheet walls by means of filling the gap between the walls with molten copper. A double-rolled tube is prepared in such a manner. Next, cold reforming and sizing are performed to obtain a final product.
As described above, double-rolled tubes generally require reliability such as air-tightness in view of their usage.
Since steel sheets used for double-rolled tubes are ultra-thin cold-rolled steel sheet having a thickness of 0.35 mm or less and require significantly excellent formability, box-annealed low-carbon steel sheets generally have been used.
Since the box-annealed sheets are relatively soft materials and have excellent formability, these can be satisfactorily used as raw materials for double-rolled tubes. The sheet, however, requires several days for production, and thus has an inferior production efficiency. Another disadvantage is non-uniformity of the mechanical properties in the longitudinal and transverse directions of the coil. In addition, in order to reduce abrasion of the die for forming the tube and in order to improve shape fixability in the tubing process (furling process), softer materials having excellent formability, while maintaining high strength, are demanded.
Ultra-low-carbon steel sheets having a significantly decreased carbon content (0.020% or less) have been noted in the field of general cold-rolled steel sheets. The ultra-low-carbon steel sheets are suitable for a continuous annealing process having a high production efficiency and creating excellent uniformity of the mechanical properties. Further, the steel sheets are soft and have excellent formability. The use of a continuously-annealed soft ultra-low-carbon steel sheet is in prospect for solving the above-mentioned problems.
In the production process of the double-rolled tube, however, a cold working of approximately 7% to 8% strain is applied to the steel sheet after tubing by drawing. Further, the tube is subjected to heat treatment for self-brazing at a higher temperature than the melting point (1,083.degree. C.) of copper, although for a short duration. Thus, coarsening of the micro-structure in the steel during the forming and annealing is anticipated. When a double-rolled tube is formed of an ultra-low-carbon steel sheet, presence of coarse grains severely affecting the strength and toughness of the double-rolled tube are often observed.
It is an object of the present invention to solve the above-mentioned problems involved in conventional technologies, and thus to provide a cold-rolled steel sheet suitable for producing double-rolled tubes having self-brazing characteristics, as well as significantly improved mechanical properties compared to conventional materials, and high production efficiency and uniformity of mechanical properties, and a method for making the same.
It is a particular object of the present invention to provide a cold-rolled steel sheet suitable for producing double-rolled tubes having the following characteristics, and a method for making the same:
1) Deterioration of its characteristics, particularly strength and toughness due to coarse grains does not occur during the heat treatment for self-brazing; PA1 2) The steel sheet has a low deformation resistance in the tube production process to minimize abrasion of a die and thus to prolong its life; PA1 3) The steel sheet is soft during the production of the tube and has excellent shape fixability; PA1 4) The final tube has sufficiently high strength, ductility, and toughness; and PA1 5) The steel sheet is an ultra thin sheet with a thickness of 0.35 mm, has excellent uniformity of mechanical properties in the longitudinal and transverse directions of the steel sheet (steel strip), and has no variation in the shape. PA1 C: 0.0005-0.020 wt %; and further comprising one or two of PA1 C: 0.0005-0.020 wt %, PA1 S: 0.02 wt % or less, and PA1 N: 0.0050 wt % or less; and further comprising one or two of PA1 C: 0.0005-0.020 wt %, PA1 Si: 0.10 wt % or less, PA1 Mn: 0.1-1.5 wt %, PA1 P: 0.02 wt % or less, PA1 S: 0.02 wt % or less, PA1 Al: 0.100 wt % or less, and PA1 N: 0.0050 wt % or less; and further comprising one or two of PA1 C: 0.0005-0.020 wt %, PA1 Si: 0.10 wt % or less, PA1 Mn: 0.1-1.5 wt %, PA1 P: 0.02 wt % or less, PA1 S: 0.02 wt % or less, PA1 Al: 0.100 wt % or less, and PA1 N: 0.0050 wt % or less; and further comprising one or two of PA1 B: 0.0005-0.0020 wt %, PA1 Cu: 0.5 wt % or less, PA1 Ni: 0.5 wt % or less, PA1 Cr: 0.5 wt % or less, and PA1 Mo: 0.5 wt % or less; and PA1 hot finish rolling of a steel material containing PA1 hot finish-rolling of a steel material at a final temperature of 1,000-850.degree. C., the steel material comprising PA1 C: 0.0005-0.020 wt %, PA1 Si: 0.10 wt % or less, PA1 Mn: 0.1-1.5 wt %, PA1 P: 0.02 wt % or less, PA1 S: 0.02 wt % or less, PA1 Al: 0.100 wt % or less, and PA1 N: 0.0050 wt % or less; and further comprising one or two of PA1 C: 0.0005-0.020 wt %, PA1 Si: 0.10 wt % or less, PA1 Mn: 0.1-1.5 wt %, PA1 P: 0.02 wt % or less, PA1 S: 0.02 wt % or less, PA1 Al: 0.100 wt % or less, and PA1 N: 0.0050 wt % or less; and further comprising one or two of PA1 B: 0.0005-0.0020 wt %, PA1 Cu: 0.5 wt % or less, PA1 Ni: 0.5 wt % or less, PA1 Cr: 0.5 wt % or less, and PA1 Mo: 0.5 wt % or less; and
The present inventors have discovered that containing a given amount or more of nonprecipitated Nb or Ti is effective for preventing the growth of grains contrary to conventional knowledge that control of the precipitates is effective, as a result of intensive experimentation and study for solving the above-mentioned problems.
Further, by controlling the annealing condition within an adequate range, as well as limiting steel components, and hot-rolling conditions, such as the final temperature at finish-rolling and the coiling temperature, the present inventors have discovered that the given amount or more of nonprecipitated Nb or Ti is secured in a nonprecipitated state, that is, a solid solution state, that the crystal grain size is controllable within an optimum range, and that mechanical properties are stabilized after heat treatment in the tube production process, and the present inventors have completed the present invention.